A wire-free digital communication system has at least one transmitter and one receiver. The information to be transmitted from a transmitter is normally initially in the form of a bit sequence. The bit sequence is mapped in the transmitter onto symbols on a complex plane, by means of a mapping device or a mapper. The complex plane is covered by a real in-phase branch (I) and an imaginary quadrature branch (Q), and the symbols correspondingly have an in-phase component and a quadrature component. The complex plane is also referred to as the symbol space, state diagram or constellation diagram. The symbols are thus also referred to as states or constellation points. The mapping rule that is used in the mapping device differs depending on the digital modulation method that is used. In general, k bits can be combined to form a symbol with M=2k states. The number of states also governs the value of a digital modulation method, that is to say the number of distinguishable values or symbols which a digital modulation method has.
After the mapping device, the symbols are filtered by means of a transmission filter in the transmitter, and are finally mixed by a mixer from baseband to the pass band. The mixer modulates the symbols onto a carrier signal at a specific carrier frequency. The transmission filter is used to produce a transmission signal with as narrow a bandwidth as possible.
The resultant transmission signal is sent from the transmitter via a radio channel to the receiver. The radio channel distorts the transmission signal and adds noise to it. In the digital communication system receiver, the signal which has been received via the radio channel is converted by an analogue/digital (A/D) converter to a quantized received signal. The quantized received signal is then down-mixed by a mixer to an intermediate frequency (IF) or to baseband. Once the signal has been down-mixed, it is necessary to ensure that only those signal components which are within the frequency band defined by the signal bandwidth are passed to the further processing stages. This is referred to as channel selection. A reception filter is normally provided for this purpose, which suppresses the undesirable frequency components, and if required together with the transmission filter reduces the intersymbol interference, and maximizes the signal-to-noise ratio.
The received signal is sampled in a sampling device after being filtered, and the symbols in the received signal are associated in a subsequent decision-making device with the symbol which has most probably been transmitted. The symbols are then converted or demodulated again by a mapping device or a demapper, on the basis of the digital modulation method that is used, to form a bit stream.
The optimum demodulation with the minimum bit error rate (BER) is based on the concept of a matched filter. Matched filters are based on the knowledge that the signal-to-noise ratio (SNR) for a radio channel with white Gaussian noise (AWGN=Additive White Gaussian Noise) can be maximized when the impulse response of the reception filter, as a matched filter, is the complex-conduit, time mirror-image impulse response of the transmission filter. Furthermore, the demodulation process must be synchronized, that is to say the carrier frequency and the carrier phase must be known or recovered in the receiver. If the carrier phase is known, this is referred to as coherent demodulation, and if the carrier phase is not known, this is referred to as non-coherent demodulation.
There are various digital modulation methods which are used in digital communication systems. The basic digital modulation methods include amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK). In the case of amplitude shift keying or modulation, the in-phase component of a symbol can assume M different amplitude values and one sampling time. The quadrature component is not used in this case. In the case of phase shift keying or modulation, M states are distinguished, with a different phase angle and the same amplitude. The points on the complex plane of the symbol space thus lie on a circle. In the case of frequency shift keying or modulation, the carrier frequency is changed as a function of the data.
There are numerous sub-variants of the basic digital modulation methods and, for example, these can also be subdivided into modulation methods without any memory and modulation methods with a memory or differential modulation methods. In the case of differential modulation methods, information is transmitted by state transitions rather than by the absolute state. The receiver therefore does not need to know the absolute phase angle and need determine only the phase change between the current symbol and a subsequent symbol. In this case, there is no need to recover the carrier phase exactly, provided that the phase error per symbol resulting from any frequency offset is small. Non-coherent demodulation can therefore be used.
One type of known phase shift keying without any memory is quaternary phase shift keying (QPSK), in which the phases of the states are each shifted through 90° with respect to one another. Known phase shift keying processes with a memory, in contrast, are by way of example DQPSK (Differential Quaternary Phase Shift Keying) and 8-DPSK (Differential Phase Shift Keying). In the case of δ/4-DQPSK, by way of example, two state diagrams that are rotated through 45° are used alternately, in which case the transitions between the states may take place only between the two state diagrams, but not within the same state diagram. Only two bits can therefore be transmitted per symbol in the case of p/4-DQPSK, despite there being eight possible states. One known frequency shift keying process with a memory is GFSK (Gaussian Frequency Shift Keying), in which suitable pulse shaping is carried out in baseband with regard to the instantaneous frequency, by means of Gaussian filters, in order to suppress crosstalk between individual frequency channels. GFSK is a special case of so-called CPFSK (Continuous Phase Frequency Shift Keying). In CPFSK, and in contrast to abrupt frequency shift keying FSK, continuous phase transitions take place between the individual symbols. In the case of CPFSK, only one fundamental frequency is used, and is deliberately mistuned. The phase angle thus remains continuous.
In wire-free digital communication systems such as DECT (Digital Enhanced Cordless Telecommunications), WDCT (Worldwide Digital Cordless Telecommunications) or Bluetooth, suitable receivers are required for wire-free reception of transmitted radio-frequency signals, these being receivers which are suitable for processing of the digital modulation type that is used in the respective communication system. In addition to high sensitivity, it is desirable for the receiver to have a high degree of integration, to cost little, to consume little power and to be flexible in terms of applicability to different digital communication systems.
The wire-free digital communication systems nowadays preferably use GFSK as the digital modulation method. In this two-value modulation method, it is preferable to use a receiver with a limiter or a limiting device instead of a conventional A/D converter. The limiting device converts the received signal to a binary data stream using a simple comparator, with a low intermediate frequency. The limiting device chops off all the input levels above a predetermined level threshold, that is to say it produces an output signal at a constant signal level in the chopped-off area. If the limiting device has high gain and/or a low level threshold, it is operated virtually continuously in the chopped-off area. A discrete-value (binary) signal, which is nevertheless continuous over time, is thus produced just by this means at the output of the limiting device. The useful information in the signal at the outputs of the limiting device is contained in its zero crossings. The rest of the signal processing is carried out digitally. The use of a limiting device is distinguished by low costs and a low power consumption, since there is no need to use a conventional high-resolution analogue/digital (A/D) converter.
The standards for the digital communication systems DECT, WDCT or Bluetooth are currently being developed further towards higher data rates, with digital modulation methods such as DQPSK and 8-DPSK with more values being used. In this case, a root-cosine filter (RRC=Root-Raised Cosine) is preferably used as the transmission filter for signal forming in the transmitter. Investigations have shown that simply adding a limiting device to a conventional receiver for a modulation method with more values leads to a severe loss of performance in comparison to a linear (ideal) receiver. This loss of performance may be quite considerable, particularly when subject to the influence of non-linear effects. Since the standards provide in addition for the capability to switch to the more robust two-value modulation methods, for example when the reception conditions are poor, the terminals have to support both the two-value digital modulation methods, and digital modulation methods with more values.
Since the performance loss that has been mentioned when using a receiver with a limiting device is unacceptable, a receiver with a conventional high-resolution A/D converter is used nowadays for the modulation methods with more values. If the digitization is carried out after quadrature signal production, as is generally the case nowadays, two A/D converters are required, in each case one for the I branch and one for the Q branch of the digital modulation method. If the resolution of an A/D converter is sufficient (for example 10 bits), this makes it possible to achieve approximately the same performance as an ideal receiver.
One disadvantage of the use of conventional high-resolution A/D converters is, however, that the implementation complexity is very high, and this is therefore associated with increased costs and a greater power consumption.